1. Field of the Invention
The present invention relates to a projection lens apparatus for a rear projection television.
2. Background
Generally, a projection television enlarges and projects on a screen three picture images of red (R), green (G) and blue (B) emitted from a cathode ray tube (CRT) through three projection lenses. There are two different techniques for projecting an image on a screen. According to a first such technique known as a front projection technique, the picture images are projected on the front surface of the screen and are reflected therefrom so that the images can be watched by a user. According to a second technique known as a rear projection technique, the picture images are projected on the rear surface of the screen and are transmitted therethrough so that the images can be watched by the user. In this latter technique, since the lights entering the screen from the surroundings are not reflected from the screen, but rather, are transmitted through the screen, the user can easily watch the picture images even in bright indoor places.
Such a rear projection television can be generally varied in regard to the size of the system and the capability of forming the image with the projection lens. The projection lens for the rear projection television requires a short projection distance (i.e., the distance between the projection lens and the screen to reduce the size of the projection system), a small F-number to form a bright image on the screen, and an excellent image formation capability to produce an excellent image.
Accordingly, to satisfy the above requirements, a number of the projection lenses have been developed. However, in such conventional projection lenses, a variety of the problems have been encountered. First, since the existing projection lens groups are comprised of three lenses for three groups, correction of aberration is limited. Accordingly, the image formation capability is limited. Also, since the projection distance is relatively long and the size of the lens is large, the size of the system cannot be reduced. Further, the ratio of the amount of light formed on the peripheral portion of the screen with respect to the amount of light formed on the central portion of the screen is not more than 30%. Therefore, the peripheral image is darker than the central image.
Also, degradation of the performance of the plastic lens, which is generally used as the projection lens, due to the peripheral temperature variation is not properly corrected. Furthermore, to correct chromatic aberration of the R, G and B lights, the distance of the projection lens from the CRT is primarily adjusted. However, such a correction is limited, thereby dimming the peripheral image on the screen. Still further, the distortion aberration in which a rectangular screen surface is distorted on the screen which is formed by the lights enlarged and projected through the projection lens, can be corrected by means of a convergence means within the CRT. However, the larger the aberration, the greater the cost associated with correcting this aberration.